The present invention provides cleaning compositions that can be used for a variety of applications including, for example, removing unwanted resist films, post-etch, BARC and post-ash residue on a semiconductor substrate. In particular, the present invention provides cleaning compositions that comprise a salt selected from a guanidinium salt, an acetamidinium salt, a formamidinium salt, and mixtures thereof as a cleaning agent.
The background of the present invention will be described in connection with its use in cleaning applications involving the manufacture of integrated circuits. It should be understood, however, that the use of the present invention has wider applicability.
In the manufacture of integrated circuits, it is sometimes necessary to etch openings or other geometries in a thin film deposited or grown on the surface of silicon, gallium arsenide, glass, or other substrate located on an in-process integrated circuit wafer. Present methods for etching such a film require that the film be exposed to a chemical etching agent to remove portions of the film. The particular etching agent used to remove the portions of the film depends upon the nature of the film. In the case of an oxide film, for example, the etching agent may be hydrofluoric acid. In the case of a polysilicon film, it will typically be hydrofluoric acid or a mixture of nitric acid and acetic acid.
In order to assure that only desired portions of the film are removed, a photolithography process is used, through which a pattern in a computer drafted photo mask is transferred to the surface of the film. The mask serves to identify the areas of the film which are to be selectively removed. This pattern is formed with a photoresist material, which is a light sensitive material spun onto the in-process integrated circuit wafer in a thin film and exposed to high intensity radiation projected through the photo mask. The exposed or unexposed photoresist material, depending on its composition, is typically dissolved with developers, leaving a pattern which allows etching to take place in the selected areas, while preventing etching in other areas. Positive-type resists, for example, have been extensively used as masking materials to delineate patterns on a substrate that, when etching occurs, will become vias, trenches, contact holes, etc.
The trend towards the miniaturization of semiconductor devices has lead to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization such as, for example, those noted below. The use of highly absorbing anti-reflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two deleterious effects of back reflectivity are thin film interference and reflective notching. Thin film interference results in changes in critical linewidth dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes. Variations of linewidth are proportional to the swing ratio (S) and therefore must be minimized for better linewidth control. Swing ratio is defined as S=4(RaRb)1/2eαD, where Ra is the reflectivity at the resist/air or resist/top coat interface, Rb is the reflectivity at the resist/substrate interface, α is the resist optical absorption coefficient, and D is the film thickness.
Bottom anti-reflective coatings (“BARC”) function by absorbing the radiation used for exposing the photoresist, thus reducing Rb and thereby reducing the swing ratio. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to linewidth variations, and in the extreme case, forming regions with complete resist loss. Similarly, dyed top anti-reflective coatings reduce the swing ratio by reducing Ra, where the coating has the optimal values for refractive index and absorption characteristics, such as absorbing wavelength and intensity.
Increasingly, a dry etching process such as, for example, plasma etching, reactive ion etching, or ion milling is used to attack the photoresist-unprotected area of the substrate to form the vias, trenches, contact holes, etc. As a result of the plasma etching process, photoresist, etching gas and etched material by-products are deposited as residues around or on the sidewall of the etched openings on the substrate.
Such dry etching processes also typically render the resist mask extremely difficult to remove. For example, in complex semiconductor devices such as advanced DRAMS and logic devices with multiple layers of back end lines of interconnect wiring, reactive ion etching (RIE) is used to produce vias through the interlayer dielectric to provide contact between one level of silicon, silicide or metal wiring to the next level of wiring. These vias typically expose, Al, AlCu, Cu, Ti, TiN, Ta, TaN, silicon or a silicide such as, for example, a silicide of tungsten, titanium or cobalt. The RIE process leaves a residue on the involved substrate comprising a complex mixture that may include, for example, re-sputtered oxide material, polymeric material derived from the etch gas, and organic material from the resist used to delineate the vias.
Additionally, following the termination of the etching step, the photoresist (including BARC or other antireflective coating) and etch residues must be removed from the protected area of the wafer so that the final finishing operation can take place. This can be accomplished in a plasma “ashing” step by the use of suitable plasma ashing gases. This typically occurs at high temperatures, for example, above 200° C. Ashing converts most of the organic residues to volatile species, but leaves behind on the substrate a predominantly inorganic residue. Such residue typically remains not only on the surface of the substrate, but also on inside walls of vias that may be present. As a result, ash-treated substrates are often treated with a cleaning composition typically referred to as a “liquid stripping composition” to remove the highly adherent residue from the substrate. Finding a suitable cleaning composition for removal of this residue without adversely affecting, e.g., corroding, dissolving or dulling, the metal circuitry has also proven problematic. Failure to completely remove or neutralize the residue can result in discontinuances in the circuitry wiring and undesirable increases in electrical resistance.
Prior art stripping compositions have included, for example: (a) organic sulfonic acid-based stripping solutions that contain an alkyl benzenesulfonic acid as the main stripping component; and (b) organic amine-based stripping solutions that contain an amine such as monoethanol amine as the main stripping component. Such prior art stripping compositions for removing the etching residue suffer, however, from significant drawbacks. For example, their use tends to erode copper wire exposed on the bottoms of via holes.
Therefore, there is a need in the art for a cleaning composition for back-end cleaning operations including stripping photoresist and plasma ash residues that is capable of selectively removing such residues without etching the underlying and exposed substrate. The underlying and exposed substrate includes, for example, metals, high dielectric constant materials (referred to herein as “high-k”), silicon, silicide and/or interlevel dielectric materials including low dielectric constant materials (referred to herein as “low-k”), such as deposited oxides, HSQ films, MSQ films, Fox films, black diamond films, and tetraethylorthosilicate (“TEOS”) films.